Distribution anisotropy: the influence of magnetic interactions on the anisotropy of magnetic remanence

The anisotropy of magnetic remanence (AMR) is often used as a tool for examining magnetic anisotropy of rocks. However, the influence of magnetostatic interactions on AMR has not been previously rigorously addressed either theoretically or experimentally, though it is widely thought to be highly significant. Using a three-dimensional micromagnetic algorithm, we have conducted a systematic numerical study of the role of magnetostatic interactions on AMR. We have considered both lineation and foliation, by modelling assemblages of ideal single domain grains and magnetically non-uniform magnetite-like cubic grains. We show that magnetostatic interactions strongly affect the measured AMR signal. It is found that depending on the orientation of the single-grain anisotropy and grain spacing it is possible for the AMR signal from a chain or grid of grains to be either oblate or prolate. For non-uniform grains, the degree of anisotropy generally increases with increasing interactions. In the modelling of AMR anisotropy, saturation isothermal remanence was chosen for numerical tractability. The influence of interactions on other types of more commonly measured AMR, are considered in light of the results in this paper. © The Geological Society of London 2004.Accepted versio

Testing the use of viscous remanent magnetisation to date flood events

© 2015 Muxworthy, Williams and Heslop. Using erratics associated with large flood events, this paper assesses whether their viscous remanent magnetisation (VRM) can be used to date the flood events. We tested this method using flood erratics from three large events: (1) the Late Pleistocene Bonneville mega-flood in Idaho, USA, (~14–18 ka), (2) the 1918 A.D. Mt. Katla, Iceland, eruption and associated jökulhaup (meltwater flood) at Mýrdalssandur, and (3) the Markarfljót jökulhaup due to an earlier eruption of Mt. Katla (~2.5 ka). We measured 236 specimens, 66 of which yielded clear identifiable and measurable viscous magnetisation signals from erratics with clustered VRM directions. From the VRM unblocking temperatures, age estimates were made. The age estimate for the most recent event (Mýrdalssandur) worked well, with a median estimated age of 80 years (with individual erratic estimates distributed between 61–105 years) compared to the known age of 91 years. The ages of the other two events were over-estimated. The estimates for Markarfljót [15 ka (7–33 ka)] were based on the results of just one erratic. For the Bonneville flood the estimates were too old, however, this locality had the largest uncertainty in the ambient temperature used in the age determination; the VRM acquired is strongly dependent on the ambient temperature, the older the event the greater the uncertainty. Southern Idaho currently has hot summers, with average summer maximum temperatures of ~31°C, but a mean annual temperature of only ~ 9°C. It is suggested that the VRM dating method works best for recent events (<2–3 ka) where the ambient temperature history can be constrained

Cooling behaviour of partial thermoremanences induced in multidomain magnetite

Accepted versio

Direct visualization of the thermomagnetic behavior of pseudo-single-domain magnetite particles

The study of the paleomagnetic signal recorded by rocks allows scientists to understand Earth’s past magnetic field and the formation of the geodynamo. The magnetic recording fidelity of this signal is dependent on the magnetic domain state it adopts. The most prevalent example found in nature is the pseudo–single-domain (PSD) structure, yet its recording fidelity is poorly understood. Here, the thermoremanent behavior of PSD magnetite (Fe3O4) particles, which dominate the magnetic signatures of many rock lithologies, is investigated using electron holography. This study provides spatially resolved magnetic information from individual Fe3O4 grains as a function of temperature, which has been previously inaccessible. A small exemplar Fe3O4 grain (~150 nm) exhibits dynamic movement of its magnetic vortex structure above 400°C, recovering its original state upon cooling, whereas a larger exemplar Fe3O4 grain (~250 nm) is shown to retain its vortex state on heating to 550°C, close to the Curie temperature of 580°C. Hence, we demonstrate that Fe3O4 grains containing vortex structures are indeed reliable recorders of paleodirectional and paleointensity information, and the presence of PSD magnetic signals does not preclude the successful recovery of paleomagnetic signals

First-order reversal curve (FORC) diagrams of nanomagnets with cubic magnetocrystalline anisotropy: a numerical approach

First-order reversal curve (FORC) diagrams are increasingly used as a material’s magnetic domain state fingerprint. FORC diagrams of noninteracting dispersions of single-domain (SD) particles with uniaxial magnetocrystalline anisotropy (MCA) are well studied. However, a large class of materials possess a cubic MCA, for which the FORC diagram properties of noninteracting SD particle dispersions are less understood. A coherent rotation model was implemented to study the FORC diagram properties of noninteracting ensembles of SD particles with positive and negative MCA constants. The pattern formation mechanism is identified and related to the irreversible events the individual particles undergo. Our results support the utility of FORC diagrams for the identification of noninteracting to weakly-interacting SD particles with cubic MCA

First-order reversal curve (FORC) diagrams for pseudo-single-domain magnetites at high temperature

Accepted versio